The invention pertains to x-ray apparatus, and more particularly to a real-time, direct viewing, x-ray image intensifier tube of the proximity type for medical x-ray fluoroscopy.
An early type of real-time, direct viewing, x-ray device is a fluoroscope. With such a device, the patient is positioned between the source of x-rays and the device. The device consists of a thick, green light emitting fluorescent screen, also known as the fluoroscopic screen which has a low resolution and a low conversion efficiency in the range of 70 ergs per square centimeter-roentgen (erg/cm.sup.2 -R) or about 10 candelasecond per square meter-roentgen (cd-sec/m.sup.2 -R). This type of x-ray apparatus, although it allows real-time, direct viewing of a full size image, and easy palpation of the patient, is no longer in popular medical use. This is because the brightness or the conversion efficiency of this system is far low than that of the well-accepted inverter type of x-ray image intensifer system. The low brightness of the old-time fluoroscopic screen forces the physicians to work in a darkened room with dark-adapted eyes. The long time (e.g., good portion of an hour) required for dark-adaptation, which was often out of proportion to the brevity of the examination itself, was a great inconvenience and a poor use of time to physicians. Furthermore, in a darkened room, the viewing condition is more strenuous, movement about the room or manipulation of the patient is more difficult, and the possibility of a group viewing is less satisfactory. Darkened room also adds unnecessary apprehension to patients.
In a well-known study by R.E. Sturm and R.H. Morgan, published in The American Journal of Roentgenology and Radium Therapy, Volume 62, (1949) pages 617-634, it was found that visual acuity and contrast discrimination were compromised by the low conversion efficiency of the old-time fluoroscopic screen. It was further stated that maximum improvement in both visual acuity and contrast discrimination may be obtained with an ideal screen intensifier at gains of 30 to 50 times (approximately 300 to 500 cd-sec/m.sup.2 -R), and gains of 500 to 1000 times (approximatley 5,000 to 10,000 cd-sec/m.sup.2 -R) are needed in practical instruments if dark adaptation is to be avoided.
The applicants have also confirmed the findings of Sturm and Morgan that a conversion efficiency in the range of 5,000 to 10,000 cd-sec/m.sup.2 -R are practical for direct viewing medical x-ray fluoroscopy.
The common present day real-time x-ray fluoroscopy is done with a television (TV) fluoroscopy system. See FIG. 1. This system uses a closed-circuit TV optically coupled to a conventional inverter type x-ray image intensifier tube which has a minified output image size. In such a system, the patient again is positioned between the source of the x-rays and the system. The conventional inverter type x-ray image intensifier tube typically has a convexly curved, six to nine inch diameter input x-ray sensitive screen which converts the x-ray image into a light image which, in turn, is converted into electrons which are then accelerated and electrostatically focused onto an output image screen which is considerably smaller than the input screen, being typically 0.6 inches to 1.0 inches in diameter. During fluoroscopy, the TV monitor is placed to one side of the patient and therefore the doctor must turn away from the patient to view the x-ray image display on the television monitor.
Some direct viewing fluoroscopic systems may be found today, which have a mirror and lens system coupled to the conventional inverter type x-ray image intensifier tube. This mirror/lens system is necessary to allow the output image to be magnified and inverted to the upright position for direct viewing. The limited exit aperture of this optical system is a great inconvenience to the physicians. The physicians's head has to follow the system around during "panning" or scanning of the patient. Also, group viewing is very difficult with this system.
The conversion efficiency of the conventional inverter type image intensifier tube used in TV fluoroscopic or direct viewing fluoroscopic systems is usually around 200,000 to 700,000 erg/cm.sup.2 -R or about 50,000 to 100,000 cd-sec/m.sup.2 -R, which is about 3,000 to 10,000 times the conversion efficiency of the old-time fluoroscopic screen. Part of this intensification is obtained as true electronic gain, or the gain at unity magnification (output size same as input size), which is about 30 to 100 times over the old-time fluoroscopic screen. Another factor of 100 gain is obtained through the 100 fold area minification of the image on the output screen. It is important to note here that without area minification gain, the conversion efficiency of this device is about 30 - 100 which is not adequate for direct viewing fluoroscopy.
The conventional inverter type x-ray image intensifier system has basic limitations in maintaining the image quality if the input field size is to expand beyond the typical nine inch diameter. The intensifier tube contains a vacuum and the electron optics of this design requires a tube length approximate to that of the tube diameter. Thus, the large vacuum space contained by the tube represents a stored potential energy which could be a major hazard in the form of a massive implosion. The electron optics of this tube demand that the input screen must be strongly curved so that all parts of the screen can be brought into focus on the output screen. This curved input screen creates spatial distortion in the image due to the projection of the x-ray shadow image onto a curve surface. Furthermore, the electron optics are such that electrons leaving different parts of the input surface experience a difference in electrical fields which results in uneven sharpness in the image from the center of the screen to the edge. Another factor is that the conventional closed circuit TV system has only 1.5 line pairs/mm limiting resolution.
The foregoing mentioned shortcomings of current fluoroscopic systems are recognized by the physicians and by the workers in the field. There had been numerous attempts at overcoming these shortcomings. The art which is closest to the invention is described below.
A recent article published by C. B. Johnson in the Proceedings of the Society of Photo Optical Instrumentation Engineers, Volume 35, pages 3-8 (1973), hypothetically suggests that an x-ray sensitive proximity type image intensifier may be designed with an x-ray sensitive conversion screen on one side of a glass support and a photocathode on the other side of the glass support. However, the article gives no specifics concerning the critical parameters or what might be used as the x-ray sensitive conversion screen. How this image intensifier can be designed to result in high conversion efficiency without the help of area minification was also not discussed.
A proximity device using a microchannel plate (MCP) both as the primary x-ray sensitive conversion screen and as an electron multiplication device was described by S. Balter and his associates in Radiology, Volume 110, pages 673-676 (1974), and by Manley et al in U.S. Pat. No. 3,394,261. According to an article published by J. Adams in Advances in Electronics and Electron Physics, Volume 22A (Academic Press, 1966), pages 139-153, this type of device has a very low quantum detection efficiency in the practical medical diagnostic x-ray energy range of 30 - 100 Kev. The device gain of the Balter article was first reported to be 20 - 30 cd-sec/m.sup.2 -R which is too low to be useful as a fluoroscopic device. A higher gain device described in the same Balter article exhibited excessive noise. There is a real question whether a practical self-supporting MCP plate with uniform gain can be constructed with current technology to sizes beyond five inches in diameter which is not of sufficient size to produce an output useful for fluoroscopic purposes.
Another approach involving proximity design was taken by I.C.P. Millar and his associates and their results were published in 1) IEEE Transactions on Electron Devices, Volume ED-18, pages 1101-1108 (1971), and 2) Advances in Electronics and Electron Physics, Volume 33A, pages 153-165 (1972).
Millar's approach again involves the use of a microchannel plate (MCP). In this device, however, the MCP is used purely as an electron multiplication device and not as an x-ray conversion screen. The conversion factor for Millar's tube is reported to be around 200,000 cd-sec/m.sup.2 -R, which is about or higher than needed for fluoroscopic purposes. However, the output brightness of Millar's tube also exhibits strong dependence on the photocathode current density. At around a photocathode current density of 5 .times. 10.sup.-11 amperes/cm.sup.2 or at the equivalent x-ray input dose rate of around 0.6 .times. 10.sup.-3 R/sec, the output brightness of the tube starts to become sublinear in response with respect to the input x-ray dose rate. The sublinear response becomes worse at higher x-ray dose rate. This undesirable feature reduces contrast discrimination during fluoroscopy. Again, it is unknown whether a large format MCP beyond five inches in diameter, self supporting and with uniform gain, can be fabricated.
The Millar proximity type image intensifier tube has a glass envelope and an inwardly concave, titanium input window. The window is described as being 0.3mm thick. Materials such as titanium, aluminum and beryllium cause undesirable scattering of the x-rays which reduces the image quality. Furthermore, because of the relatively high porosity and low tensile strength properties of such materials, they cannot be made as thin as desirable to maximize their x-ray transmissive properties as windows for a high vacuum device. Still another problem with tubes constructed with such materials for the input window and glass for the tube envelope is in joining the window of sufficiently large area to the tube envelope. The materials have such dissimilar thermal expansion properties, among other differences as to preclude their practical commercial use in a large format device.
As is suggested by the foregoing description of prior art direct x-ray viewing attempts, the problems of designing a proximity type x-ray image intensifier tube which is both convenient to use and is of sufficient gain and resolution are highly complex in their interrelationships. For example, one way to achieve high gain with a proximity device is to increase the high voltage applied between the scintillator-photocathode screen and the output display screen. Unfortunately this is limited by the problem of field emission, which is indeed pointed out by Millar and others. By increasing the spacing between the scintillator-photocathode screen and the output display screen, could allow increase in voltage, but as Millar pointed out, this also has the effect of greatly deteriorating the image quality due to electrostatic defocusing.
Another problem of prior art direct x-ray image viewing attempts is in minimizing the patient dosage while maximizing the x-ray image information content at the scintillator-photocathode screen. If the scintillator screen is made thicker, to thereby be more efficient in stopping x-rays, it also adds "unsharpness" to the picture. This would be unacceptable in the conventional inverter type x-ray image intensifier tube and optical viewing system because there are already many other sources of "unsharpness" such that the image quality of the total system is just barely acceptable.